It is well known that carbon nanotubes are single- or multi-walled cylindrical structures, in which each of the layers of the cylindrical wall is a graphite-like sheet of carbon atoms (graphene).
Carbon nanotubes have a complex of unique properties due to their chemical and structural characteristics including the small size of the diameter, cylindrical structure and high form-factor (the ratio of the length of a carbon nanotube to the diameter thereof). Carbon nanotubes are characterized by extraordinary high strength (about 150 GPa), Young's modulus (about 600 GPa), low density (about 2 g/cm3), high chemical stability, thermal and electrical conductivities.
Important geometric characteristics of carbon nanotubes are a number of carbon monomolecular layers/walls, an outer diameter, a diameter of the inner hole, a length, a form-factor.
Main methods for producing carbon nanotubes are arc, laser, electrolysis and catalytic methods. It is commonly used in industry the catalytic method which enables to use a relatively simple equipment, provide a continuous synthesis mode, produce high-yield carbon nanotubes (Ando Y., Zhao X., Sugai T., Kumar M. Growing carbon nanotubes//Materials Today, 2004, pp. 22-29). The essence of the method consists in that a carbon-bearing gas (carbon precursor) is decomposed over a metallic catalyst at the temperature of 500° C. to 1500° C. The process is performed by one of two methods: by growing nanotubes on a substrate or in a gas stream (Mordkovich V. Z. Ultrahigh-strength carbon nanofibers//Chemical industry today, 2003, No. 2, pp. 12-21).
Carbon nanomaterials, in particular fibers on basis of carbon nanotubes, are among the most promising materials for various applications, namely for using in the manufacture of sensors, displays, lithium-carbon batteries for computers and cell phones, starting capacitors for electronics, biomaterials, sorption materials and hydrogen storage systems. However, carbon nanotubes have the most actual application in development of structural and functional composite materials for different purposes and high-strength and high-modulus carbon complex filaments. The main problem of using carbon nanotubes at the macroscopic level lies in their limited length. Therefore, a development of a method for growing long carbon nanotubes (not less than several millimeters) is a necessary condition for the appearance of the corresponding class of structural and functional materials.
It is known in the art a method for producing bundles of long oriented nanofibers (RU Patent No. 2393276, published on Jun. 27, 2010), consisting in that a carbon nanofiber growth catalyst, after its high-temperature pretreatment, is introduced in a reactor, the reaction zone is heated to the temperature of pyrolysis of a carbon-bearing steam-gas mixture fed in the reactor and comprising promoters on the base of sulfur- and oxygen-containing compounds, the reaction zone is held at the temperature of pyrolysis till said bundles are formed, then the reactor is cooled. A linear feed velocity of the carbon-bearing steam-gas mixture is in the range from 20 to 300 mm/s. The high-temperature pretreatment of the catalyst is carried out in a stream of air or inert gas at the temperature of 1200 to 1300° C., the pyrolysis temperature is in the range from 1000 to 1150° C., and the carbon-bearing gas mixture is a mixture consisting of hydrogen, aromatic compounds and paraffins and/or olefins, where a volume of paraffins and/or olefins is less than 30% of the overall volume of gases. The main disadvantage of this method is that the process is not continuous. Moreover, not all the nanofibers composing bundles are nanotubes because not all of them have a cylindrical structure.
The technically closest to the claimed method is a method for producing of long single-walled carbon nanotube strands by catalytic decomposition of n-hexane containing 0.45 wt % of thiophene as a promoter in a vertical flow reactor, wherein the catalyst (ferrocene) is introduced in the form of suspension in liquid hydrocarbon (WO/2003/072859, IPC C01B 31/02, 2003). Disadvantages of the closest method are limited possibility of continuous removal of the obtained nanotubes from the reactor because the nanotubes are immobilized in the form of a “flexible smoke” at the bottom portion of the reactor and can be removed only by drawing and twisting, and also a single-walled structure of obtained materials, which makes difficult their further chemical and thermal treatment required for producing high-strength composite carbon fibers and composite materials using the obtained materials. Moreover, the known method does not provide sufficient quality of the obtained product because the nanotubes of above 5 cm in length are hardly oriented in the resulting strands, i.e. nanotubes are not sufficiently parallel in the strands and even tangled.
The closest to the claimed apparatus is an apparatus for producing carbon nanotubes, comprising means for introducing a carbon-bearing component, a promoter and a precursor of a carbon nanotube growth catalyst into a carrier gas stream to form a mixture of these components; a vertical reactor having a working chamber, means for heating the working chamber to operating temperature, means for delivering said mixture to the working chamber of the reactor and means for removing products from the working chamber (WO/2003/072859, IPC C01B 31/02, 2003). This known apparatus has the same disadvantages as the closest method.